This invention relates to the field of marine antifoulants. It is particularly directed to a coating for marine structures such as ship hulls, sonar domes, condensers, piping, buoys, and other objects submerged in saltwater which discourages attachment thereto of inorganic and organic aggregates, followed by bacteria or algae and barnacles.
The coating is in the form of a thin film containing a piezoelectric polymer, which, when electrically activated, vibrates at its interface with water containing these organisms over frequency ranges and amplitudes inhospitable to their attachment whereby subsequent foulant build-up is minimized.
The problems of marine fouling on ship hulls and marine structures by organisms present in saltwater are long standing. The application of lead and copper cladding on ship hulls was an early attempt to prevent barnacle formation. More recent attempts at solving the problem are made by the application of toxic paints and coatings containing chemicals, such as, for example, cuprous oxides, sometimes with the addition of mercurial and other compounds, which slowly leached into the water for poisoning waterborne microorganisms. Examples of these and other efforts are discussed in U.S. Pat. Nos. 3,167,473; 3,684,752; 3,979,354; 4,075,319; and 4,082,709.
It has been found that the attachment of organisms, in particular barnacles, on a marine surface follows a definite progression. It is thought that the surface is preconditioned by the collection of a film of marine slime. Because of physico-chemical forces of the surface, organic and inorganic aggregates adhere to it. These aggregates attract the bacteria and/or algae which will in a very short time colonize the surface. These bacteria are of the sliming type and thusly secrete mucoid-type compounds. These materials form bridges binding the organisms to the surface. Once the slimming is established, pioneer species of macrofouling groups settle on it. These could be barnacles, tubeworms, hydroids, bryozoans and others, depending on the geographical area and time of year. After this stage of the fouling sequence, many other organisms attach to complete the fouling community. In this community of foulants the organism larvae are living free swimming in water. This stage is of short duration, and the larvae soon seek a suitable place for settlement and attachment. If the place of settlement is inhospitable, the larvae swim away to try attachment at another location. The entire development from free-swimming larvae to young barnacles takes only about 72 hours. It is during this period that the present invention is most effective.
The resultant effect of a concentration of plants and animals settling and attaching themselves to ships are well known. They contribute significantly to increased skin frictional resistance of the hull with resulting speed reduction and increased fuel consumption. This problem of marine growth (fouling) applies not only to vessels but also to other submerged objects. For example, fouling of sonar domes has been found to seriously limit the active and passive modes of operation of ship's acoustical systems especially by generating noises. Fouling of moored data acquisition systems by marine organisms impedes operations and necessitates frequent maintenance. Buoys shift due to the excessive weight of fouling organisms. Wood pilings in berthing facilities undergo structural weakening and ultimate destruction due to marine borer and fungal attack. The fouling of piping, piping couplings and fittings in the sea water intake piping systems including condensers, leads to reduced flow rates, valve seat damage, and accelerated metal corrosion. Concrete or ferro-cement or other similar structures are also adversely affected.